Furan and related compounds are useful starting materials for industrial chemicals for use as pharmaceuticals, herbicides, stabilizers, and polymers. For example, furan is used to make tetrahydrofuran, polytetramethylene glycol, polyether ester elastomers, and polyurethane elastomers.
Known transition metal catalyzed, vapor phase processes to produce furan by decarbonylation of furfural are limited by either the selectivity or lifetime of the supported catalyst. The conversion of furfural to furan is complicated by the tendency to form polymeric or carbonizing byproducts which foul the catalyst surface and hinder the rate and lifetime of the catalyst.
Supported palladium catalysts are known to catalyze furfural decarbonylation reaction with high selectivity but are limited by short lifetime. For example, U.S. Pat. No. 3,007,941 teaches a process for the production of furan from furfural comprising heating a liquid phase consisting essentially of furfural in the presence of palladium metal and a basic salt of an alkali metal; the basic salt is not part of the catalyst per se but is continuously added to the liquid phase during the reaction. The process suffers from quick catalyst deactivation and difficult catalyst regeneration processes. U.S. Pat. No. 3,223,714 teaches a continuous low pressure vapor phase decarbonylation process for the production of furan comprising contacting furfural vapor with a supported palladium catalyst. A preferred catalyst has about 0.3 wt % Pd supported on alumina. The catalyst can be regenerated in situ but the lifetime of a running cycle for the catalyst is short and the production of furan per cycle is low. In U.S. Pat. No. 4,780,552, furan is prepared by decarbonylation of furfural in the gas phase at elevated temperatures and under from 0.1 to 10 bar in the presence of hydrogen and a catalyst which contains platinum and/or rhodium and contains an alkali metal. The reaction is carried out in the presence of hydrogen. Catalysts which contain platinum and/or rhodium and to which cesium has been added are preferably used. Using a cesium-promoted Pt/alumina catalyst results in longer lifetime than without cesium. However, moderate selectivity and the high cost of platinum metal decrease the economic viability of this process. Patents SU1699601 and RU 2027714 teach the use of cesium-promoted Pd/C catalysts to produce furan from furfural. However, the catalyst is deactivated by carbon formation on its surface, and the usual in situ carbon removal regeneration processes can not be applied because of the risk of destroying the structure of the carbon support. This limits the overall efficiency of the process.
There remains a need for catalysts for the vapor phase decarbonylation of furfural to furan with improved lifetime and with the capability of catalyst regeneration in situ.